A family of Microsoft word processing software products for creating web, email, and print documents.
Sorry, there is no longer any desktop version of Office that can be activated on your computer. Both Microsoft and Apple seem to expect you to throw out perfectly good computers because they got tired of supporting them.
As alternatives to Office for Mac under Big Sur, you can use Word for the web or Google Docs. For desktop software, take a look at open-source LibreOffice, which has good compatibility with Office file formats.
If you're adventurous, Open Core Legacy Patcher allows you to run newer versions of macOS on older Macs. You can install up to Sequoia (Tahoe is not supported). I have a 2011 MacBook Pro running Sequoia and the current edition of Office.